This invention relates to methods for testing an electronic memory, and has particular reference to methods for testing such memories when they are subjected to repeated assaults.
Electronic memories can be assaulted in any of a number of ways. They may be subjected to repeated bursts of radiation, whether of electrons, photons, or other particles, which tends to cause bit-flips by secondary electron emission. They may be subjected to repeated thermal shocks, or to repeated mechanical shocks. Other assaults will occur to those skilled in the art. Regardless of the nature of the assault, it is desirable to know how well the memory will operate when subjected thereto. "Operate" in this sense comprises two facets. First, the memory should be able to retain, after the assault, the information which had been stored in it before the assault; that is, there should be no bit-flips. Second, the memory should remain functional; that is, it should be possible, both during and after the assault, for information to be written to the memory, and to be read from it.
There is yet a third facet to memory testing. It is not enough that the memory have tested well immediately after production. Once a memory has been assaulted in the field, it is desirable to be able to retest the memory and to accurately determine whether or not it has at least retained its functionality. There may be some memories, for example, which appear to have recovered their functionality shortly after being assaulted, but which then lose their functionality after this brief interval period. If a field test of functionality is to be made, it must be done so that it does not pick up this false window of functionality, and incorrectly determine that no permanent damage has been done when actually only temporary damage has been avoided. These windows of functionality can be determined during factory testing.
The prior art in memory testing has been to load a memory with information, to subject the memory to a hostile environment for an extended period of time, and to then examine the contents of the memory for retention, and to attempt to read from and write to the memory to see how well it still functions. This is an appropriate test when, under field conditions, the memory will be subjected to hostile conditions, but not to repeated assaults. The prior art has had to simulate repeated assaults by making the environment excessively harsh, but nonetheless steady. Alternatively, if a memory was subjected to repeated assaults, it was not tested after each assault, but only after the series of assaults. This testing, while valuable, can be improved upon.